One of the persistent challenges faced in the development of semiconductor technology is the desire to increase the density of circuit elements and interconnections on substrates without introducing spurious interactions between them. Unwanted interactions are typically prevented by providing gaps or trenches that are filled with electrically insulative material to isolate the elements both physically and electrically. As circuit densities increase, however, the widths of these gaps decrease, increasing their aspect ratios and making it progressively more difficult to fill the gaps without leaving voids. The formation of voids when the gap is not filled completely is undesirable because they may adversely affect operation of the completed device, such as by trapping impurities within the insulative material.
Common techniques that are used in such gapfill applications are chemical-vapor deposition (“CVD”) techniques. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. Plasma-enhanced CVD (“PECVD”) techniques promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes when compared with conventional thermal CVD processes. These advantages may be further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive. While each of these techniques falls broadly under the umbrella of “CVD techniques,” each of them has characteristic properties that make them more or less suitable for certain specific applications.
HDP-CVD systems form a plasma that is at least approximately two orders of magnitude greater than the density of a standard, capacitively coupled plasma CVD system. Examples of HDP-CVD systems include inductively coupled plasma systems and electron cyclotron resonance (ECR) plasma systems, among others. HDP-CVD systems generally operate at lower pressure ranges than low-density plasma systems. The low chamber pressure employed in HDP-CVD systems provides active species having a long mean-free-path and reduced angular distribution. These factors, in combination with the plasma density, contribute to a significant number of constituents from the plasma reaching even the deepest portions of closely spaced gaps, providing a film with improved gapfill capabilities compared with films deposited in a low-density plasma CVD system.
Another factor that allows films deposited by HDP-CVD techniques to have improved gapfill characteristics is the promotion of sputtering by the high density of the plasma, simultaneous with film deposition. The sputtering component of HDP deposition processes slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gapfill ability of HDP deposited films. Some HDP-CVD systems introduce argon or a similar heavy inert gas to further promote the sputtering effect. These HDP-CVD systems typically employ an electrode within the substrate support pedestal that enables the creation of an electric field to bias the plasma towards the substrate. The electric field can be applied throughout the HDP deposition process for further promotion of sputtering and to provide better gapfill characteristics for a given film.
It was initially thought that because of their simultaneous deposition/sputter nature, HDP-CVD processes could fill the gaps or trenches that were created in almost any application. Semiconductor manufacturers have discovered, however, that there is a practical limit to the aspect ratio of gaps that HDP-CVD processes are able to fill. For example, one HDP-CVD process commonly used to deposit a silicon oxide gapfill film forms a plasma from a process gas that includes silane SiH4, molecular oxygen O2, and argon Ar. It has been reported that when such a process is used to fill certain narrow-width high-aspect-ratio gaps, the sputtering caused by argon in the process gas may hamper the gapfill efforts. Specifically, it has been reported that material sputtered by argon in the process redeposits on the upper portions of the sidewalls of the gaps being filled at a rate faster than at the lower portions. This, in turn, may result in the formation of a void in the gap if the upper areas of regrowth join before the gap is completely filled.
FIG. 1 provides schematic cross-sectional views of a silicon oxide film at different stages of deposition to illustrate the potential gapfill limitation associated with some CVD processes. The gapfill problem is illustrated in somewhat exaggerated form to illustrate the problem better. The top portion of FIG. 1 shows the initial structure 104 in which a gap 120 is defined by two adjacent features 124 and 128 having horizontal surfaces 122, with the horizontal surface at the bottom of the gap being denoted 132. As shown in structure 108, i.e. the second portion of the figure from the top, a conventional HDP-CVD silicon oxide deposition process results in direct deposition on the horizontal surface 132 at the bottom of the gap 120 and on the horizontal surfaces 122 above the features 124 and 128. It also, however, results in indirect deposition (referred to as “redeposition”) on the sidewalls 140 of the gap 120 due to recombination of material sputtered from the silicon oxide film as it grows. In certain small-width, high-aspect-ratio applications, the continued growth of the silicon oxide film results in formations 136 on the upper section of the sidewall 140 that grow towards each other at a rate of growth exceeding the rate at which the film grows laterally on the lower portions of the sidewall. This trend is shown in structures 108 and 112, with the final result in structure 116 being the formation of a void 144 within the film. The probability of forming a void is very directly related to the rate and character of the redeposition.
A variety of techniques have been developed to extend the gapfill capabilities of silicon oxide HDP-CVD processes. Two specific examples include U.S. Pat. No. 5,872,058 (“the '058 patent”) and U.S. Pat. No. 6,395,150 (“the '150 patent). The '058 patent discloses that the gapfill capabilities of a silicon oxide film may be extended by reducing the amount of argon or other inert components in the HDP process. This is intended to reduce the amount of sputter and thereby reduce the rate of redeposition. The '150 patent discloses that if argon, which is a diluent gas in addition to a sputtering agent, is eliminated from the process gas as suggested in the '058 patent, deposition rate uniformity may suffer. The '150 patent then teaches that this problem may be overcome by substituting a flow of argon with a flow of helium.